The invention pertains to a process for the controlled feeding of a melting crucible with particles during the drawing of a crystal by the Czochralski method by regulation of the flow rate of particles from a source to the melting crucible. The source is equipped with a conveying device for discharging particles in an adjustable quantity per unit time, and where a crystal with a constant diameter and a uniform dopant concentration, formed from the particles, is drawn from the melting crucible.
When crystals are drawn by the Czochralski method (CZ method), a crystal is drawn at a constant rate from a melt, which consists of doped silicon held in a heated crucible. The solid/liquid phase boundary is located near the surface of the melt. This method can be carried out in a discontinuous manner; that is, the entire amount of material required for the drawing of a single crystal, including the required dopant, is loaded into the melting crucible at the beginning, and the drawing process is terminated after most of this material has been consumed. The demand for crystals of greater length and of greater diameter is increasing, however, and thus there is a trend toward the use of continuous methods, in which the crucible is fed with a continuous supply of the doped starting material. This method is referred to as the CCD method (=continuous Czochralski drawing method).
In the method of drawing from the melt, a discontinuity occurs at the solid/liquid phase boundary with respect to the concentration of the dopants, which can include phosphorus, arsenic, antimony, and boron, depending on the desired characteristics of the materials to be obtained; that is, the amount of dopant in the drawn, solid material of the crystal is significantly less than that in the melt remaining behind in the crucible. In a discontinuous or batch process, this discontinuity leads to an increasing accumulation of dopants in the melt, as a result of which the concentration of the dopant obviously increases over time, so that the dopant concentration in the most recently drawn part of the crystal is much higher than that in the part of the crystal drawn first. The electrical resistance decreases over the length of the crystal, however, in proportion to the increase in the amount of dopant. Because this effect also increases as the crystal grows longer, continuous processes have been developed, in which the effect described above is compensated by adjusting the concentration of the dopants in the material being used to replenish the melt.
It is already known in the continuous Czochralski method that the dopants can be added to the melting crucible in the form of thin rods, which are fed continuously into the melt. The process used to obtain these rods, however, is extremely complicated, and so far it has proved impossible to obtain rods in which the desired concentration of dopants remains uniform over the entire length of the rod. In addition, the extreme brittleness of these rods makes it difficult to handle them.
A system for controlling the flow of pourable particles into the melting crucible of a crystal drawing device as a function of the weight of the particles is known from European Patent No. A1-537,988. Relatively large quantities of undoped Si base material are involved here, however, and this Si material constitutes the largest share by far of the total weight of the finished crystal. An intermediate container with an outlet tube and two shutoff valves is set up underneath a main storage bin, and a vibratory conveyor with a receiving container, which rests on a sensitive weight-measuring cell, is installed underneath the intermediate container. When the weight of the material in the receiving container falls below a predetermined value, he shut off valves of the intermediate container are opened in succession, so that the pourable particles continue to slide down into the receiving container until an upper weight limit is reached. The material is thus replenished intermittently, and it is stated that the weight-dependent control at the time of refilling the receiving container is initiated by the input of a value stored in a central processor. Nevertheless, a certain range of variation in the stream of material being conveyed cannot be prevented, because the amount discharged from the vibratory conveyor is necessarily subject to fluctuations over time.
The attempt has already been made to supply the melt with a stream of particles which have been produced by scratching a grid of squares into a standardized wafer and then by breaking the wafer into block-shaped particles with a square outline. Particles of this type are supplied in a continuous stream, that is, without any gap between them, to a detent mechanism, the detent pawl of which is designed to release individual particles at predetermined time intervals so that they can fall freely into the melt. This method of producing individual particles and releasing them in a controlled manner makes use of constraining forces of a purely mechanical nature and is based on the condition that the particles conform to extremely high standards of dimensional uniformity, which are hardly realizable in practice, and leads during actual operations to problems involving high levels of abrasion, contamination, clogging, and the fracture of the particles. As a result, jamming and clogging occur on the route to the detent mechanism and especially in the detent mechanism itself. The problems are attributable in part to the irregular shape of the particles but also in part to the different shapes of the fragments or splinters, etc., with the result that these experiments were finally abandoned.
The invention is therefore based upon the task of providing apparatus of the general type described above, which can be operated over extended periods of time without breakdowns and by means of which a precisely predetermined quantity of particles can be conveyed per unit time. The task to be accomplished in particular is to introduce a precisely predetermined quantity of dopants per unit time into the melt present in the melting crucible.